1 Technical Field
This disclosure relates generally to an engine starter designed to apply torque to a ring gear of an engine.
2 Background Art
There is a need to rotate a pinion gear of a starter at high speed to shorten the time required to start an internal combustion engine. When the pinion gear is rotated at high speed, it requires an electric motor of the starter to be increased in size in order to produce a degree of torque great enough to start the engine in cold conditions.
In order to meet the above requirement, Japanese Patent First Publication No. 2004-218627 proposes a starter which is designed to change a speed reduction ratio of an output shaft of an electric motor to a pinion gear of the starter and works to increase the speed reduction ratio to elevate torque outputted from the electric motor as needed. Specifically, the starter is equipped with a planetary gear train, a cam mechanism, and a housing in which the planetary gear train and the cam mechanism are installed. The planetary gear train includes a sun gear joined to an output shaft of an electric motor, planet gears meshing with the sun gear, an internal gear meshing with the planet gears, and a carrier training the planet gears to be rotatable. The cam mechanism consists of a stationary cam plate secured to the internal gear, a movable cam plate facing the stationary cam plate, balls disposed to be rollable in cam grooves formed in the stationary and movable cam plates, and an elastic member urging the movable cam plate into constant contact with the stationary cam plate.
The electric motor of the starter outputs a small degree of torque until the pinion gear meshes with the ring gear of the internal combustion engine, so that the movable cam plate is kept pressed against the stationary cam plate. This causes an inner peripheral tapered surface of the movable cam plate to be pressed against an outer periphery of an outer cam joined to the pinion gear, so that the outer cam and the internal gear are fixed, thereby establishing a gear reduction ratio of one in the planetary gear train.
Afterwards, the pinion gear meshes with the ring gear. The electric motor is then required to increase the output torque for rotating the ring gear. The movable cam plate is disengaged from the stationary cam plate, so that the outer periphery of the outer cam is separated from the inner peripheral tapered surface of the movable cam plate, and an outer peripheral tapered surface of the movable cam plate is pressed against a tapered surface of the housing. This holds the internal gear from rotating and results in an increased reduction speed ratio in the planetary gear train.
The above starter encounters the following drawback. When the starter is required to achieve a high speed reduction ratio, but, the movable cam plate has failed to establish engagement with the housing through pressurized contact between the outer peripheral tapered surface of the movable cam plate and the inner peripheral tapered surface of the housing, there is a probability that the internal gear rotates in a direction opposite to rotation of the sun gear, thus resulting in a failure in transmitting torque outputted from the motor to the pinion gear to start the engine. Additionally, when a depth of engagement of the pinion gear and the ring gear is small when the pinion is rotated in high speed, it may result in breakage of the pinon gear or the ring gear.